Measuring the velocity of bullets



' w. L. ARLsoN E-rAL MEASURING THE VELOCITY 0F BULLETS MayA 14, 1946.

Filed May v25, 1944 2 Sheets-Sheet 1 im Y :wmv/daaraan swam/MmWarmer/Pansau-'r ro :eure/e oFLooR endelLL Mme/serum alsmme Hmadafr rocE/wzo/-Laae #fram/fr Maly 14, 1946.

w. L. CARLSONETAL- 2,400,189 MEASURIKNGl -"THE lvEJLocTY oF BULLETSFiled May 25., 1944 2 Sheets-Sheet 2 u mma/ds 01u00@ summum# @grams-m 1aan I l s Zi Patented May 14, 1946 2,400,189 MEASURING THE .VELOCITY orBULLETS Wendell L. Carlson, Vernon D. Landon, and Robert L.Harvey,-Princeton, N. J., `assignors to Radio Corporation of America, acorporation of Delaware Application May 25, 1944, serial No. 537,304

' (C1. loi-1s) 15 Claims.

The present invention. relates to the generation of electrical impulsesand more particularly to such impulses caused by the flight of a`bullet. l

Some of the objects of the present invention are: to provide a novelsystem for Ameasuring the velocity of bullets; to provide a velocitymeasuring system wherein the motion of a bullet through a magnetic fieldgenerates an electrical impulse capable of being used as a measure ofthe velocity of the bullet; Ato provide a velocity measuring systemwherein a loop energized by a direct current is responsive to a bullettraversing the loop to create an impulse usable with other adjuncts ofthe system to measure the velocity of the bullet; to provide a multipleturn loop forming apath for a bullet, which loop has a lengthapproximately equal to its mean radius; to provide a velocity measuringsystem wherein a plurality of loops are provided for the successivepassage of a bullet in iiight, each loopr having an impulse generatedtherein as the y bullet passes so that a time factor is obtained as ameasure of the velocity of the bullet; to provide a system for measuringthe velocity of bullets wherein the loops are employed for successivelyresponding to the passage of a bullet plus suitable ampliiier circuitsfurnishing an output to an impulse counter; to provide a system formeasuring velocity of bullets whereinr loop and amplifier circuits areemployed responsive to either magnetic or non-magnetic bullets, plusprovision for changing circuits for operation with the type of bulletunder consideration; to provide a system for measuring the velocity ofbullets wherein electrical circuits are protected from high 4inductionvoltage; to provide a system for measuring the velocity of bulletswherein provision is made for sound proong the apparatus to obviateerrors otherwise due to vibrations incident to firing the gun orotherwise; to provide a system for measuring the velocity ofbulletswherein loop input and amplifier circuits including three impulseresponsive loops are employed, two of which are alined in the path ofthe bullet and the third coupled to the amplifier circuit; to provide adevice for measuring bullet velocity in which are combined a largeorifice to'make it easy to shoot through and in which acoustic shieldingprevents microphonics; and to provide otherimprovements as "willhereinafter appear. i

In the accompanying drawings, Figure 1 represents a diagram of thesystem of the present invention as assembled for operation; Figure 2represents one form'of circuit embodying the invention; Figure 3represents a perspectiveof one of the loops as mounted for operation;1ig'` ure l represents an end elevation of anyone of the loops; Figure 5represents a section on line V-V of Fig. 3; Figure 6 is a graphrepresenting the theoretical curve of the current induced in anon-magnetic bullet; `Figure 7 is a graph representing the theoreticalAvoltage induced in a loop by ai non-magnetic bullet; Figure 8 is agraph representing the theoretical response for non-magnetic bulletafter differentiation: and

yFigure 9 is a graph representing the theoreti-l cal loop voltage for amagnetic bullet not permanently magnetized.

Referring to the drawings, onel formv of the present inventioncomprises` three loops l0, Il and I2, two of which, I0 and I I, arearranged in spaced coaxial relation and encircle the path of a bulletfired from a gun I3. rThe third loop I2 I serves as a coupling coil fortransmitting the induced pulse to av suitable circuit for amplificationto a value -capable of actuating an electronic counter or otherelectrical responsive device. In the present instance, all of the loopsare wound, dimensioned and constructed'alike, though this is only arequirement as to the loops I0 and II, as itis unnecessary that loop `I2so conform since other coupling means may be employed. Thus, each loopconsists ola four-layer coil, live inches long and is wound of No.10double cotton covered wire. The shape of the winding is maintained byimpregnating the ,Y coil with heavy wax so that it has the conformationof a hollow cylinder.

For protecting and sound proofing the loop coils, each is housed by twocoaxial tubes of insulating material I4 and I5, having ends closed bywooden rings I6 connected to the cardboard thereby to form a'sealedhousing around the coil. As a support for' the coil a plurality offeltrings I1, such as "Ozite encircle the coil within the rings i6, while afelt ring I8 encircles the outer face of the coilto fill the spacebetween this face andthe inner wall of the outer tube I4. It should beparticularly lnoted that the inner face of the coil is spaced from theinner tube I5 to form an annular air chamber which insulates the coilfrom sound vibrations. Thus, by acoustically shielding the coil, themicrophonic response of the loop is reduced to a minimum. Two relativelyheavy terminals 20 and 2| lare secured to the outer tube Il, and arerespectively connected by ilexible conductors 22 and 23 to the ends oithe coil.

The ratio of coil diameter and sound proof housing inner diameter shouldbe as near unity as practical so as to provide a large area for thepassage of the bullet. Therefore, the coil is supported mainly.from itsouter surface through thick sound absorbingmaterial. The air spacebetween the inner surface of the coil and the housing should be reducedto a minimum determined by mechanical clearances.

In order toprovide a substantiallyvuniform magnetic field within thecoil and. therefore. a

` The uniform response from bullets passing through the center or oi! toone side of center, we find that the coil width should be approximatelythe'same v dimension as the coil radius. Also, we ilnd that thisconstruction minimizes self-induced voltages set up in the coil due tovibration. 'If the coil turns are concentrated, i. e., more layers andfewer turns per layer, the magnetic field will be` less' uniform, andthe microphonics will be increased.

Itis important that -theelectricalgcenterof each coil be accuratelylocated and this point is' obtained in the following manner. The threeloops are coaxially locatedon a wooden rack with a spacing of abouteighteen inches between the loops. The two outer loops are connected inseries, bucking and across an audio signal generatcr. The middle loop isconnected to the input of an audio amplier (that shown in Fig. 2 may theresistance of Rx.

dierentiating part of the circuit consists of the capacitor Cxand theresistor Rx. Over a range of frequencies containing the pulse com--ponents. the reactance of Cx is much larger than Thus the current isalmost entirely a function of Cx. 'I'his current is proportional to therate of change of the signalvoltage. Hence. output is the derivative ofthe input voltage. The gain is adjusted by proper circuit constants sothat the output, when. using standard 0.30 calibre magnetic ornon-magnetic ammunition, will be constant. A five-step volume control.circuit with atotal range of 100 to 1 follows the switching circuitand' feeds the grid of the second ampliner tube ll; The anode circuit ofthe tube Il consists of an output transformer, the secondary of which isdesigned to work into a balanced load of 2000 ohms with the centergrounded. The entire amplifier has, essentially, a flat be used) and theoutput of the amplifier is connected to an audio indicating device. Withabout 2000cycle output from the generator, one ofthe end loopsis'adjusted laterally to obtain a good null.A From a fixed pointer,situated near the center of the loop under test, a line is marked on theloop housing. Now, without moving the pointer or .other coils, themarked coil is rotated through 180 degrees,.the leads reversed, anothernull obtained, and a second mark made on the coil housing. Equi-distantbetween these two marks is the electrical center of the coil.k ThisismarkedbyanarrowasshownonFig. 3.

For mounting the loops, each coil assembly includes a support 24 vintheform'of a sturdy v wooden rackbuilt entirely of non-metallicmaterial. Thus, allparts of the support 24 are held togetherbywaterproof glue and dowels. Finally. j

the support is waterproof-treated and finished a dull black to presentacontrast to the bright yellow loop housing. Preferably, rubber pads forma bearing for the loop housing and the Weilht ofthe loop will besufiicient to hold itin place. Should the weight be -foundinsu'fllcient, suitable non-metallic means can be wrapped around thevhousing and support for greater mechanical stability. Prom theforegoing it will v be seen that there are three independent supportswith coil housings respectively thereon, two of which are to beinstalled on the firing range in proper position while the third is.located in the gun room near the control panel.

Referring to ll'igs.v l and 2. the loops il, Il and I2 are in a seriescircuit supplied with current from a source 2l, in thisinstance astorage bat' tery, and the `loops Il and I I are coaxially alined inspaced relation, while the loop Itis in coupling lresponse from 100 to10,000 cycles. 'I'he relay 2l is energised or deenergized according tothe position of a controlswitch 3| and indication .of

which relay circuit-has been selected is evidenced by a red pilot lampil in one circuit and a greenv pilot lamp 88 in the other circuit. Aswitch 34 controls current to the indicating lamps and the relay. ywhilea switch controls current from the source-ll to the loops. The couplingcircuit also includes aeswitch It which when open prevents high lvoltagesurge from the'loop circuit fromv while all damaging the transformer.Thus, switches are closed when the system is operating, the vswitch Ilvis closed last when starting operation and is opened first whenstopping operation. As shown in l'lg. 1, the loop circuits, battery,

. relay. and output. are associated with a control `source.

Inoperation the loops Il andll are located on the range where they arecarefully adjusted for the distance between them, namely by measuringfrom black arrow on one loop housingfto the black arrow on the otherloop housing.V These arrows indicate theelectrical center o: the maps.with all circuitsy closed. abullet is fired through the two coaxialloops il and. Il, thereby cutting the D.C. magnetic fields of the`energized loops, producing eddy currents in the bullet which in-v ducea voltage pulse in each loop. Since the loops are spaced a measured andknown distance apart, the velocity of the bullet can now be calculatedusing the -time measure.

All calculations vare based upon the mutual inductancebetween the bulletand the loop as a function of the time or distance. In thesecalculations solutions for Pulse shapes are required for non-magneticand forl magnetic'bullets with and WithOllt DTB-meditation. The 'bulletsavailable for trial were all 0.30 calibre and include y `thelcopper-iticketed. lead-core bullet; the steellack'eted, lead-corebullet; and the armorpiercing relation with an input transformer 2l as astepup means to the grid of the firstvamplifier tube 2l. This tube 21 ispreferably one which isfree from microphonic disturbance and its anodeworks into a network which is arranged to be switched by means of. arelay" from a differentiating circuit for non-magnetic bullets tp a ,lowgain amplifier circuit for magnetic bullets.

`bullet which haslfa hardened steel core and a copper Jacket, The.approximate solutions for the pulse shapes for the various bullets havebeen found and have been checkedby taking oscillovgrams.A Thetheoreticaipulseswere foundto quite closely to the actual pulses. Thus.in lllg. l the theoretical curve of the current induced in thenon-magnetic bulletis shown.

interval between pulses as a ascenso for energizing said loops to createa magnetic field in each, and means responsive to an electrical pulsegenerated in each loop by the passage of a bullet through `the coilmagnetic iield 'for indicating the presence of said bullet, whereby thesuccessive pulses produce a time interval from which the velocity of thebullet can be calculated.

2. An apparatus for measuring the velocity oi bullets, comprising twoloops arrangedin coaxial spaced relation to form a path for a bullet,each loop being formed by a multiple turn coil of wire, means forenergizing said loops to create a magnetic field in each, an audioamplifier, means responsive to an electrical pulse generated in eachloop by the passageof a bullet through the loop magnetic iield forindicating the presence of said bullet, whereby the successivepulsesproduce a time interval from which the velocity of the bullet can becomputed, means coupling said responsive means to said audio ampliiier,and means to prevent high induction voltage surge in said coupling meansduring the initial functioning of said energizing means.

3; An apparatus for measuring the velocity of bullets, comprising twoloops arranged in coaxial spaced relation to form a path for a bullet,each netic iield in each, an audio ampliiier, means` responsive to anelectrical pulse generated in each loop by the passage oi a bulletthrough the l loop magnetic iield for indicating the presence of saidbullet, whereby the successive pulses produce a time interval from whichthe velocity of the bullet can be computed, means coupling saidresponsive means to said audio amplifier, and means rendering saidcoupling means ineffective during the initial functioning oi saidenergizing means.

4. An apparatus for measuring the velocity of bullets, comprising twoloops arranged in coaxial spaced relation to form a path for a bullet,each loop being formed by a multiple turn coil of wire, means providingan acoustic shielding for each loop, said shielding means forming anannular air space encircled by its loop, means for energizing said loopsto create a magnetic eld-in each, an audio amplifier, means responsiveto an electrical pulse generated in each loop by the passage of a bulletthrough the loop magnetic field for indicating the presence of saidbullet, wherebythe successive pulses produce a time interval from whichthe velocity of the bullet can be computed, and means couplingsaidresponsive means to said audio ampliiier.

5. An apparatus for measuring the velocity of bullets, comprising twoloops arranged in coaxial spaced relation to form a path for a bullet,each loop being formed by a multiple turn coil of wire, means providingan acoustic shielding for each loop, said shielding means forming anannular air space encircled by its loop, means for energizing said loopsto create a magnetic eld Ain each, an audio amplier, means responsive tolan electrical pulse generated in each loop by the passage of a bulletthrough the loop-magnetic field for indicating the presence of saidbullet, whereby the successive vpulses produce a time interval fromwhich the velocity of the bullet can be computed, means coupling saidresponsive means to said audio ampliiier, and means to prevent highinduction voltage surge in said coupling means during the ,initialfunctioning of 'said energizing means.

6. An apparatus for measuring the velocity of Y bullets', comprising twoloops arranged in coaxial spaced relation to form a path for a bullet,each loop being formed by a multiple turn coil oi' wire, means providingan acoustic shielding for each loop, said shielding means forming anannular air space encircled by its loop, means for energizing said loopsto create a magnetic field in each, an audio amplifier, means responsiveto an electrical pulse generated in each loop bythe passage of a bulletthrough the loop magnetic field for indicating the presence of saidbullet. whereby the successivepulses produce a vtime interval from whichthe velocity of the bullet can be computed, means coupling saidresponsive means to said audio ampliiier, and means rendering said4coupling means ineffective during the initial functioning of saidenergizing means.

7. In an apparatus for measuring the velocity of bullets, comprising twoloops arranged in coaxial spaced relation to form a path for a bullet,each loop being formed as a multiple turn loop having a lengthapproximately equal to its mean radius, means providing an acousticshielding for each loop, said shielding means forming anannular airspace encircled by its loop, means for energizing said loop to create amagnetic field in each, and means responsive to an electrical impulsegenerated in each loop by the passage of a bullet through the loopmagnetic iield for indicating the presence of said bullet, whereby thesuccessive pulses produce a time interval from which the velocity of thebullet can be calculated.

8. In an apparatus for measuring the velocity -of bullets, comprisingtwo loops arranged in coaxial spaced relation to form a path for abullet, each loop being formed as a multiple turn loop, direct currentmeans for` .energizing said loops to create a magnetic field in each,means comprising an audio amplifier and a coupling loop responsive to anelectrical impulse generated in each alined loop by the passage of abullet through the loop magnetic iield for indicating the presence ofsaid bullet, whereby the successive pulses produce a time interval fromwhich the velocity of the bullet can be calculated.

' 9. In an apparatus for measuring the velocity of bullets, comprisingtwo loops arranged in coaxial spaced relation to form a path for abullet, means comprising a coupling loop responsive to an electricalimpulse generated in each alined loop by the passage of a bullet throughthe loop magnetic field for indicating the presence of a bullet, wherebythe successive pulses produce afv time interval from which the velocityof the bullet can be calculated, two electrical circuits constituting anaudio amplifier and energized by said coupling means, one of saidcircuits functioningv for non-magnetic bullets and the other functioningfor magnetic bullets, and means for selecting the circuit correspondingto the bullet used.

l0. In an apparatus for measuring the velocityof a bullet, a loop formedby a multiple turn 4 coil, a shield enclosing said loop to absorb soundwaves, said shield forming an annular spacel on one side of said coiland on the other side a through hole for the passage loi' a bullet, andterminals leading from the coilv to the exterior o! said shield. v

1l. In an apparatus for measuring the velocity oi' a bullet, acylindrical loop formed by a multiple turn coil having a lengthapproximately equal to its mean radius and forming an annular space onits outer side with the inner periphery oi' said loop and on its innerface a through .hole forthe passage of a bullet, a sound proofhousingabout said coil, and terminals leading from Vsaid coil to the exerior ofsaid housing.

12. A device as in claim 9 in which the audio ampliiier circuit usedyfor non-magnetic. bullets differs from that used for magnetic bulletsby the inclusion of a differentiating circuit.

13. In an apparatus for measuring the velocity of a bullet,a-loopforxned by a multiple turn coil, an enclosure of sound-deadeningmaterialrabout the outer periphery of said loop, a tube of insulatingmaterial encircling said material. a tube of y insulating materialspaced from said coil to form an air space at one side and a throughhole at the other side for the passage ot a bullet, and two non-magneticcoverings respectively closing the end spaces oi said tubes.

14. In ank apparatus for measuring the velocity ot a bullet, acylindrical loop formed by a multiple turn coil, a ring ofsound-deadening ma.

terialaboutthe outer periphery of saidl loop, a tube of. insulatingmaterial encircling said material, a tube` of insulating material spacedcircumferentially from said coil to form an air space atone side and athrough hole at the other side for the e of a bullet, two non-magnetic srings respectively closing the end spaces `of 'said tubes, andAsound-d'eadening Arings respectively betweensaid coil and saidnon-magnetic rings.

15. In an apparatus for measuring the velocity of a bullet, acylindrical loop formed by amul'-y tiple turn coil having a lengthapproximately equal to its mean radius, a ring of sound-deadenlngmaterial about the outer periphery o!r said loop, a tube of insulatingmaterial encircling said material, a tube oi' insulating material spacedcir- 4 cumferentially from said coil to form an air space at one sideand a through hole at the other side for the passage of a bullet, andtwo non-magnetic

